Hydrocarbons are one of the main sources for the manufacture of syngas. The manufacture of syngas consists in converting hydrocarbons CnHm to a mixture of at least hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2). The gases produced are then used to carry out numerous chemical reactions. Thus, the hydrogen could be used, in particular, for carrying out hydrogenation reactions or, after addition of nitrogen N2, for producing ammonia; a mixture of CO, CO2 and H2 could result in the synthesis of methanol, and mixtures of CO and H2 are the basis for oxo syntheses, etc.
One of the most used methods for achieving this conversion is the catalytic steam reforming of light hydrocarbons, in particular natural gas. In such a method, the mixture of light hydrocarbons, mainly comprising methane, is reacted with steam in the presence of a catalyst to produce hydrogen and carbon oxides. Among the various reactions carried out during the reforming, the main reactions are: the endothermic reforming reaction, namely CH4+H2OCO+3H2, and also the exothermic CO conversion reaction, namely CO+H2OCO2+H2. Reforming is overall endothermic and is carried out at a temperature generally in the vicinity of 1000° C. It should be noted that the hydrocarbons are, in general, previously stripped of the sulfur compounds that they contain, the latter being poisons for the catalysts commonly used.
One method for the catalytic steam reforming of light hydrocarbons is carried out in a combustion chamber comprising burners and tubes, the tubes being filled with catalyst and being capable of being passed through by a mixture of hydrocarbons and steam, the burners being arranged so as to transfer the heat from their combustion to the mixture of hydrocarbons and steam through the wall of the tubes.
In practice, the catalytic reforming reaction of hydrocarbons by steam is carried out under pressure over the catalyst contained in the tubes that are heated externally by radiation and convection. The catalyst tubes are positioned vertically and the circulation of the reaction mixture of hydrocarbons and steam is carried out from the top to the bottom.
A reforming furnace comprises a radiation zone, combustion chamber in which the catalyst tubes are placed, and a convection zone, via which the evacuation of the flue gases and of the combustion gases produced in the combustion chamber is carried out. The combustion gases evacuated through the convection zone are used for preheating the incoming reaction mixture of hydrocarbons and steam, and optionally other reaction fluids. The convection zone is generally installed either on top of the combustion chamber, or vertically to the side of the furnace, or horizontally. Depending on the position of the burners in the furnace (at the top for top-fired furnaces, or on the side for side-fired furnaces), the vertical temperature profile of the tube is different. The maximum temperature will especially be achieved at a point of the tube located in around its upper third in the case of a top-fired furnace and around the lower third in the case of a side-fired furnace.
The catalyst tubes used are generally of the centrifugally cast alloy steel tube type. The tube must withstand the high pressures and temperatures used in such a method, while ensuring a good transmission of the heat to the reaction mixture which circulates inside. In addition to having to possess a strength adapted to the very high temperatures, these tubes must be made from a material that also has a very good creep resistance at such temperatures.
The catalyst tubes are subjected to heating/cooling cycles and to very high temperatures; their aging and their integrity must be monitored as accurately and reliably as possible. Specifically, they are calculated for a finite service life (typically 100 000 hours) at a maximum operating temperature DTT (Design Tube Temperature). Exceeding this maximum limit value leads to a significant reduction in the service life of the tubes. For example, constant operation at 20° C. above the operating temperature for which the tubes were designed halves the service life of the tubes, changing it from around ten years to around five years. This problem takes on a most particular importance during start-up phases or phases of changing the composition of the reaction mixture. Monitoring of the temperatures to which the tubes are subjected is therefore essential, but the knowledge of the history of these temperatures is also of crucial significance for the catalytic hydrocarbon reforming method.
Currently, measurements of the temperature of the tubes are carried out discontinuously, in particular by pyrometric measurements. Measurements may also be carried out by using, in particular, thermocouples. Such measurements, whether they are pyrometric or via thermocouples are isolated, both in space and time; they do not make it possible to know all of the temperature differences to which each catalyst tube is subjected throughout its life and over its entire length.
Furthermore, the pyrometric measurements are carried out via peepholes made in the wall of the furnace. When carrying out a measurement, the operator opens the peephole and points the pyrometer (or another measuring instrument) horizontally towards the tube whose wall temperature must be measured; this manipulation tends however to reduce the temperature inside the furnace level with the peephole.
In the case of thermocouple measurements, it should be noted that the thermocouples are an integral part of the catalyst tubes and therefore the temperature measurements do not require intervention by an operator unlike pyrometric measurements. The temperature perturbations caused by an operator opening a peephole are avoided as are the risks incurred by the operator during such a measurement. However, it is not always possible to implant the thermocouple at the location of the tube where the temperature is supposed to be at a maximum since at this location its presence could create a hot spot.
One of the main risks that stems from the absence of reliable and accurate data on the actual temperature of the tubes and the variations undergone is the risk of premature aging leading to the untimely rupture of one or more catalyst tubes in the course of operation and to the shutdown of the plant—outside of predefined maintenance schedules.
In particular, during transient phases such as a restart following a maintenance shutdown or a change in the composition of the reaction mixture, there is a risk of overheating not being detected and corrected immediately; the service life of the catalyst tubes may then be significantly reduced. Consequently, since the risk of overheating cannot be eliminated, it is essential to control the aging of the tubes in order to program the change thereof as soon as it becomes necessary.
Reliable monitoring of the temperatures of each of the tubes is all the more important since the temperatures of the tubes in operation are not the same depending on their position in the furnace, but also as a function of various factors, such as the ignition sequences, the types of hydrocarbons and fuels used, etc.